In recent years, a number of polymers have been developed, which are biocompatible, that is, do not cause tissue irritation or necrosis, platelet adhesion, or erythrocyte lysis.
One of the existing materials, polyvinyl chloride (PVC), has been widely used in blood tubing and blood bags; however, PVC is a rigid thrombogenic material which requires a plasticizer which may cause problems; e.g., some are suspected of being carcinogenic and others may alter the composition of the blood.
Biocompatible materials for contact with blood or tissue also have been made from block copolymers comprising alternate "hard" and "soft" segments. The hard segments confer the requisite physical properties, while the soft segments confer the desired biological properties upon the biocompatible materials. Since these segmented copolymers exhibit elasticity, the need for plasticizers, such as found in single component polymers, is avoided.
For example, commercial materials have become available in which the biocompatible material is a segmented polyether polyurethane [BIOMER.RTM. from Ethicon] and polydialkylsiloxane polyurethanes [AVCOTHANE.RTM. from Avco Medical Products]. Furthermore, polyester/polyether block copolymers having blood compatibility and therefore useful for shaping into biologically useful products have been described in Jones et al. U.S. Pat. Nos. 3,908,201 and 4,042,978 and Toyobo Japanese Pat. No. 53-10586.
While the segmented copolymers described above constituted a major advance in the state of the art, they, too, are deficient in some properties. For example, it is difficult to extrude the segmented urethanes into shapes from a melt.
Polyethylene oxide is widely employed as the polyether in both types of segmented copolymers, but when present in useful amounts, it is a crystalline material, having an attraction for blood platelets. It would appear from data reported by Decosta et al., JOURNAL OF COLLOID AND INTERFACE SCIENCE, VOL. 72, August 1980, pp. 594-596, that under the conditions under which their data was obtained, the polyethylene oxide segment (PEO) in the segmented polyurethanes is amorphous at low molecular weights; however, high molecular weight PEO is crystalline at greater than about ten percent (10%) concentration.
To use pure PEO, even at low concentrations, in blood contact applications, requires that the PEO must have a molecular weight of less than 2,000; otherwise, the soft segment will become crystalline. This limitation directly affects the physical properties of the polymer, because, for a given composition, the longer the chain length of the soft segment, the longer the length of the hard segment will be. Increasing the length of the hard segment enhances physical cross linking in these materials, especially in polyester based elastomers, which physical cross linking directly contributes to the strength and dimensional stability of these polymers.
The statement made above that the longer the soft segment, the longer the hard segment will be, can be demonstrated by reference to a given segmented copolyester composition having a fixed ratio of hard to soft segments (such as one which is 40% by weight polyester and 60% by weight polyester). Since (for a given composition) the mole fraction of a soft segment decreases by increasing its molecular weight, the use of higher molecular weight soft segments results in longer hard segments.